Dialkylnaphthalenes, such as 2,6-dimethylnaphthalenes, are desirable starting materials for oxidation to the corresponding naphthalene dicarboxylic acids. Such acids are monomers that are useful for the preparation of a variety of polymers. For example 2,6-naphthalene dicarboxylic acid when polymerized with ethylene glycol produces poly(ethylene 2,6-naphthalate) which can have better heat resistance and mechanical properties than polyethylene terephthalate, and which can be used to make films, fibers, and the like.
In refinery streams derived from petroleum or coal, dimethylnaphthalenes are present as isomeric mixtures which are difficult and costly to separate. Hence, techniques for producing specific dimethylnaphthalenes alone or in admixture with one or two other specific isomers are highly desirable. One promising process is to catalytically cyclize one or more alkenylbenzenes into the corresponding dialkyltetralins, i.e., dialkyltetrahydronaphthalenes, which are then dehydrogenated to the corresponding dialkylnaphthalenes. A mixture of such dialkylnaphthalenes, such as a mixture of triad of 1,6-, 1,5-, and/or 2,6-dimethylnaphthalenes, can then be isomerized to a single such product, such as 2,6-dimethylnaphthalene. Such a cyclization, dehydrogenation, and isomerization process sequence is described, for example, in commonly assigned Sikkenga et al., copending U.S. Patent application Ser. No. 316,308 filed Feb. 27, 1989.
The catalyst heretofore employed for the cyclization of the alkenylbenzenes in such process usually is an acidic, ultrastable, Y-type zeolite catalyst in hydrogen form, having a unit cell size of about 24.2 to about 24.7 Angstroms, and SiO.sub.2 to Al.sub.2 O.sub.3 molar ratio of about 4:1 to about 6:1, and a sodium content of from about 0.05 to about 3.5 weight percent, calculated as elemental sodium and based on the weight of the zeolite. It has now been found, however, that as less acidic catalyst of the same general type, i.e., one having a substantially higher SiO.sub.2 to Al.sub.2 O.sub.3 molar ratio and a low Na.sub.2 O content provides an unexpectedly improved cyclization of alkenylbenzenes in higher yields and with improved selectivity.